Hi afatliberal,
The NHTSA?s National Center for Statistics and Analysis does have
their 2005 report available online, which includes 2004 data. The
number reported is the same as my collegue found below.
Traffic Safety Facts - 2005 Data
http://www-nrd.nhtsa.dot.gov/pdf/nrd-30/NCSA/TSF2005/AlcoholTSF05.pdf
..."Injuries and Arrests
In 2004, the Federal Bureau of Investigation?s Uniform Crime Reporting Program
estimated that over 1.4 million drivers were arrested for driving under the
influence of alcohol or narcotics. This is an arrest rate of 1 for
every 139 licensed drivers in the United States. (2005 data not yet
available.)..."
Watch this web page for the 2006 report:
National Certer for Statistics and Analysis
of the National Highway Traffic Safety Administration
http://www-nrd.nhtsa.dot.gov/departments/nrd-30/ncsa/AvailInf.html
Hope this helps!
~~Cynthia
Search strategy:
I clicked on the link my collegue provided, and followed the citation
given by MADD and found a more recent report. Also gave client source
page to watch for 2006 report.

I don't have the full answer for you, but here are the results from
2003--the most current year of available statistics.
"Approximately 1.4 million drivers were arrested in 2003 for driving
under the influence of alcohol or narcotics. This is an arrest rate of
1 for every 135 licensed drivers in the United States (2004 data not
yet available). (NHTSA, 2005)" from http://www.madd.org/stats/1784

Important Disclaimer:
Answers and comments provided on Google Answers are general
information, and are not intended to substitute for
informed professional medical, psychiatric, psychological, tax,
legal, investment, accounting, or other professional
advice. Google does not endorse, and expressly disclaims
liability for any product, manufacturer, distributor,
service or service provider mentioned or any opinion
expressed in answers or comments. Please read carefully the
Google Answers Terms of Service.

If you feel that you have found inappropriate content,
please let us know by emailing us at
answers-support@google.com
with the question ID listed above. Thank you.